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Comparison of Filter Bag, Cyclonic, and Wet Dust Collection Methods in Vacuum Cleaners

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In this study, methods were developed for comparative evaluation of three primary dust collection methods employed in vacuum cleaners: filter bag, cyclonic, and wet primary dust collection. The dry collectors were evaluated with KCl test aerosols that are commonly used in filter testing. However, these aerosols cannot be used for evaluating wet collectors due to their hygroscopicity. Therefore, the wet collectors were evaluated with nonhygroscopic test particles. Both types of test aerosol indicated similar collection efficiencies in tests with dry collectors. The data show that high initial collection efficiency can be achieved by any one of the three dust collection methods: up to 50% for 0.35 microm particles, and close to 100% for 1.0 microm and larger particles. The degree of dependence of the initial collection efficiency on airflow rate was strongly related to the type and manufacturing of the primary dust collector. Collection efficiency decreased most with decreasing flow rate for the tested wet collectors. The tested cyclonic and wet collectors showed high reentrainment of already collected dust particles. After the filter bag collectors had been loaded with test dust, they also reemitted particles. The degree of reentrainment from filter bags depends on the particulate load and the type of filter material used. Thus, the overall particle emissions performance of a vacuum cleaner depends not only on the dust collection efficiency of the primary collector and other filtration elements employed, but also on the degree of reentrainment of already collected particles.
Content may be subject to copyright.
Copyright 2001, AIHA
AIHAJ 62:573–583 (2001) Ms. #257
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AIHAJ (62) September/October 2001 573
A
UTHORS
Saulius Trakumas
a,c
Klaus Willeke
a
Tiina Reponen
a
Sergey A. Grinshpun
a
Warren Friedman
b
a
Aerosol Research and Exposure
Assessment Laboratory,
Department of Environmental
Health, University of Cincinnati,
P.O. Box 670056, Cincinnati,
OH 45267–0056;
b
Office of Lead Hazard Control,
U.S. Department of Housing
and Urban Development, 451
7th St. SW (P 3206),
Washington, DC 20410;
c
Current address: SKC Inc., 863
Valley View Road, Eighty Four,
PA 15330; E-mail:
SKCSaulius@SKCinc.com
Comparison of Filter Bag, Cyclonic,
and Wet Dust Collection Methods
in Vacuum Cleaners
In this study, methods were developed for comparative evaluation of three primary dust
collection methods employed in vacuum cleaners: filter bag, cyclonic, and wet primary dust
collection. The dry collectors were evaluated with KCl test aerosols that are commonly used in
filter testing. However, these aerosols cannot be used for evaluating wet collectors due to their
hygroscopicity. Therefore, the wet collectors were evaluated with nonhygroscopic test particles.
Both types of test aerosol indicated similar collection efficiencies in tests with dry collectors.
The data show that high initial collection efficiency can be achieved by any one of the three
dust collection methods: up to 50% for 0.35
m
m particles, and close to 100% for 1.0
m
m and
larger particles. The degree of dependence of the initial collection efficiency on airflow rate was
strongly related to the type and manufacturing of the primary dust collector. Collection
efficiency decreased most with decreasing flow rate for the tested wet collectors. The tested
cyclonic and wet collectors showed high reentrainment of already collected dust particles. After
the filter bag collectors had been loaded with test dust, they also reemitted particles. The
degree of reentrainment from filter bags depends on the particulate load and the type of filter
material used. Thus, the overall particle emissions performance of a vacuum cleaner depends
not only on the dust collection efficiency of the primary collector and other filtration elements
employed, but also on the degree of reentrainment of already collected particles.
Keywords: collection efficiency, cyclone, emission, filter bag, lead-based paint abatement,
vacuum cleaner, wet collector
This research was supported
by the U.S. Department of
Housing and Urban
Development, Office of Lead
Hazard Control, grant nos.
OHLHR0026–97 and
OHLHR0054–99.
V
acuum cleaners are commonly used for
regular cleaning of surfaces in industrial
and commercial buildings, in homes, and
for special purposes such as lead-based
paint hazard control cleanup.
(1,2)
Dust from the
surface being cleaned is picked up through the
nozzle of the vacuum cleaner, and most of it is
captured by the dust collection components in-
stalled in the vacuum cleaner. Some of the dust
may penetrate through the primary dust collec-
tors and will then be expelled to the ambient air
or be captured by the final high efficiency par-
ticulate air (HEPA) filter, if installed. The
amount of dust that penetrates through the vac-
uum cleaner depends on the efficiency of the
dust collection components installed in the de-
vice. Use of a less efficient dust collector leads to
a higher dust emission level, and vice versa. Thus,
the dust removal efficiency of a vacuum cleaner
affects the indoor environmental quality after
vacuum cleaning.
(3–5)
It has been shown that household and in-
dustrial vacuum cleaners with a final HEPA fil-
ter installed in the exhaust airflow initially re-
move close to 100% of 0.3 mm and larger
particles.
(6–8)
The lifetime of the expensive final
HEPA filter depends on the performance of the
primary dust removal element of the vacuum
cleaner: a less efficient primary collector will
cause higher dust loading on the final HEPA
filter.
(8)
Thus, the efficiency of the primary dust
collector affects the loading of the final HEPA
filter in the vacuum cleaner and its replacement
frequency during use.
The three principal methods used for pri-
mary dust removal in vacuum cleaners are dust
collection in a disposable filter bag (filter bag
collector), dust removal by centrifugal motion
574 AIHAJ (62) September/October 2001
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(cyclonic collector), and dust removal by impingement into wa-
ter (wet collector). Once a filter bag is filled with collected dust,
it is disposed of and replaced by a new one, typically costing $1
to $3.
(9)
No such replacement cost is incurred with cyclonic and
wet collectors. In a cyclonic collector the collected dust is re-
moved from the chamber; in a wet collector the soiled water is
replaced by fresh tap water. Because the effluent airflow from a
wet dust collector is humid, the standard test techniques for eval-
uating dry dust collectors cannot be used.
The test techniques and procedures developed and employed
in this study permit direct comparisons among the three dust col-
lection methods. To do so, the initial collection efficiencies were
measured and compared for filter bag, cyclonic, and wet dust col-
lectors. Dust reentrainment from these collectors was also evalu-
ated after initial loading of each collector with the same amount
of test dust.
EXPERIMENTAL MATERIALS AND METHODS
Filter Bag, Cyclonic, and Wet Dust Collection in Vacuum Cleaners
Each vacuum cleaner is equipped with a primary dust collector
that removes and collects most of the dust from the airstream
going through the device. One or more additional filtration ele-
ments may be installed in the vacuum cleaner for further dust
removal and protection of the air mover components from dust.
The purpose of the final HEPA filter, if installed, is to assure that
virtually no particles are emitted to the ambient air environment.
Figure 1 schematically shows the three principal dust collection
methods employed in vacuum cleaners.
The filter bag (Figure 1A) is the most commonly used primary
dust collector in vacuum cleaners.
(10)
Usually, filter bags are made
from fibrous filter media. According to filtration theory, particles
in the airstream may deposit on the fiber surfaces due to diffusion,
interception, inertial impaction, or gravitational settling.
(11,12)
The
contribution of each of these filtration mechanisms to the overall
filtration efficiency depends on parameters such as particle size,
filter material, and the airflow velocity through the filter.
(11,12)
Ac-
cumulated dust on a filter medium may increase the pressure drop
across the filter and thus affect the filtration characteristics.
(11–13)
Therefore, a loaded filter bag must be replaced with a new one.
The filter bags available from the manufacturers have different fil-
tration efficiencies. A vacuum cleaner collects dust more efficiently
when a filter bag with higher efficiency is installed,
(8)
unless the
higher efficiency bag significantly reduces the airflow through the
vacuum cleaner. Filter bags are widely used in canister and upright
vacuum cleaners.
Recently, more companies have marketed vacuum cleaners with
cyclonic dust collection. A typical cyclonic dust collector is sche-
matically shown in Figure 1B. It is also used in either canister or
upright vacuum cleaners. In this type of collector, the dust con-
taining airflow is drawn into a cylindrical chamber, in which it
swirls downward and then leaves the chamber upward through a
central tube.
(14,15)
Swirling particles with sufficient inertia are de-
posited onto the inner surface of the cylinder due to the inertial
(centrifugal) forces on them. The efficiency of particulate collec-
tion depends on such parameters as the airflow rate through the
device, the size of the cylinder, and the dimensions of the inlet
and outlet tubes.
(15,16)
Periodically, the collected dust is removed
and the surfaces of the cyclone are cleaned.
The third method of dust collection in vacuum cleaners is im-
pingement into water (Figure 1C). It appears that only canister-
type vacuum cleaners are available with this type of collector. In a
wet collector, particles are impacted into a reservoir filled with
water.
(14,17,18)
As in all inertial collection devices, the velocity of the
airflow and particle size are the most important parameters.
(15)
A
mist separator is usually installed above the wet collector to pre-
vent droplets from the bubbling water to affect the performance
of the air mover and motor. As with cyclonic collectors, wet col-
lectors do not include elements that need to be replaced period-
ically with new ones, except the water, after it has become dust-
laden.
Description of the Vacuum Cleaners Tested
Six different brands of household vacuum cleaners were tested in
this study, two each of the filter bag, cyclonic, and wet dust col-
lection types. The characteristics of these devices are summarized
in Table I. The labeling for the type of motor placement was
introduced and schematically shown in a previous publication.
(8)
Type II indicates that the air mover is placed after the primary
dust collector. In Type IIa the motor emissions are combined with
the effluent airflow from the primary dust collector, whereas in
Type IIb the motor emissions are separate from the effluent air-
flow coming from the primary dust collector. In previous studies
five different filter-equipped vacuum cleaners were evaluated.
(7,8)
Two of these were used for the present comparison tests with
cyclonic and wet collectors. To help the reader desiring more in-
formation on filter-containing vacuum cleaners, the labeling for
the filter collectors (FC) in the present publication is the same as
in the previous publications.
Vacuum cleaner FC3-UP (ca. $160) was an upright vacuum
cleaner with a filter bag as the primary dust collector. The filter
bag had about 2000 cm
2
(;2.2 ft
2
) in filtration surface, and con-
sisted of three layers of fibrous filter material. The motor was pre-
ceded by a small prefilter. A final HEPA filter captured the motor-
emitted particles and the dust particles not removed previously by
the filter components. The maximum flow rate through this de-
vice, Q
IN
, was 60 ft
3
/min, when operated with all filters installed.
In vacuum cleaner FC4-CAN (ca. $650), the filter bag collector
was installed in a canister. It also contained a small motor prefilter
and a final HEPA filter. The filter bag had about 1400 cm
2
(;1.5
ft
2
) in filtration surface and consisted of a single layer of fibrous
material. Additional information on the performance of these two
vacuum cleaners can be found in previous publications.
(7,8)
Two vacuum cleaners with cyclonic collectors (CC) were eval-
uated in this study: upright CC1-UP (ca. $170) and canister CC2-
CAN (ca. $300). Vacuum cleaner CC1-UP contained a chamber
for the collection of large dust particles and a cyclone. The un-
collected particles were removed in one of the subsequent dust
collectors: a cyclone afterfilter, a small motor prefilter, and a final
HEPA filter. The HEPA filter also removed the particulate motor
emissions. Vacuum cleaner CC2 contained a dual cyclone, a fine
metal grid for motor protection, and a final HEPA filter for re-
moving the remaining dust particles and the particulate motor
emissions.
In both wet collectors (WC) tested in this study ($1200–
$1400), the water container was placed in a canister. In vacuum
cleaner WC1-CAN the container was filled with 1.9 L (2 quarts)
of tap water. Water droplets in the effluent air were removed by
a mist separator before entering the air mover. Particles passing
out of the wet collector were captured by a final HEPA filter. An
additional filter removed particles from the motor emissions. Vac-
uum cleaner WC2-CAN employed 3.8 L (1 gallon) of water. A
mist separator was also installed before the air mover. A small final
AIHAJ (62) September/October 2001 575
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FIGURE 1. Schematic of the three principal dust collection methods used in vacuum cleaners. The final filters on some vacuum cleaners, including
the ones used in this study, are HEPA filters. PDC
5
primary dust collector.
TABLE I. Characteristics of Tested Vacuum Cleaners
Label Category
Primary
Collector
Type
Motor
Placement
Type
A
Final
HEPA
Maximum
Flowrate,
Q, ft
3
Pressure Drop,
D
P
PDC OUT
,
B
inch H
2
O
FC3-UP
FC4-CAN
CC1-UP
CC2-CAN
WC1-CAN
WC2-CAN
upright
canister
upright
canister
canister
canister
filter bag
filter bag
cyclone
cyclone
wet
wet
IIa
IIa
IIa
IIa
IIb
IIb
yes
yes
yes
yes
yes
none
60
80
50
40
62
56
23
27
32
54
16
18
A
Test labels correspond to those used in the previous publication ‘‘Particle Emission Characteristics of Filter-Equipped Vacuum Cleaners’’ by S. Trakumas, K. Willeke,
S.A. Grinshpun, T. Reponen, G. Mainelis, and W. Friedman,
AIHAJ 62
:482–493 (2001).
B
DP
PDC OUT
5 P
PDC OUT
2 P
AMBIENT
.
filter (not HEPA), installed after the air mover, collected previ-
ously uncollected particles. The motor emissions were separate
from the effluent airflow coming from the primary dust collector
and were not filtered.
Data presented in the last column of Table I show the pressure
drop at the outlet of primary dust collectors tested, DP
PDC OUT
5
P
PDC OUT
2 P
AMBIENT
. The lowest pressure drop was registered
at the outlet of wet collectors WC1 and WC2. The value of
576 AIHAJ (62) September/October 2001
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DP
PDC OUT
measured for cyclonic collectors was 2 to 3 times higher
than the pressure drop at the outlet of wet collectors. The values
of pressure drop across the filter bags appear to be between the
ones measured for wet and cyclonic collectors, respectively.
Test Methods
Measuring the Initial Collection Efficiency of Different
Primary Dust Collectors
The primary dust collectors of six different vacuum cleaners were
first evaluated as to their initial collection efficiency. The term ini-
tial reflects the collection efficiency of a clean dust collector; that
is, when new filter bags are installed in the filter collectors, all dust
is removed from the cyclonic collectors, and clean water is put
into the wet collectors.
The initial collection efficiency of the primary dust collectors
(PDC) was measured through probed testing.
(7,8)
Identical probes
were installed at the primary dust collector inlet and outlet, as
shown in Figure 1. The aerosol concentrations in the airflow en-
tering the primary dust collector, C
PDC IN
, and leaving it, C
PDC OUT
,
were simultaneously measured with optical particle size spectrom-
eters (model 1.108, Grimm Technologies, Douglasville, Ga.). The
vacuum cleaner was connected through a hose (no nozzle was
used) to a clean air supply system
(7)
and was operated for 30 min
before each test. During the next 10 min, the background aerosol
concentration was registered in the airflow leaving the primary
dust collector, while there was no test aerosol input. The aerosol
generator was then activated, and concentrations C
PDC IN
and
C
PDC OUT
were measured three times during a 4-min period. The
collection efficiency, E, of the primary dust collector was calculat-
ed by Equation 1:
C 2 C
PDC OUT BACKGROUND
E 5 1 2 100%
(1)
12
C
PDC IN
The average collection efficiency and standard deviation were cal-
culated from three measurements of C
PDC IN
and C
PDC OUT
.
As indicated earlier, the dust collection efficiencies for the filter
bag, cyclonic, and wet collectors depend on the airflow rates
through them. When a vacuum cleaner is used in dusty environ-
ments, the airflow through it can decrease due to loading with
dust particles on the different dust removal components. The air-
flow through a vacuum cleaner also depends on the type of nozzle
used and the characteristics of the surface being cleaned.
(8)
To asses
how the airflow rate affects the collection efficiency of the differ-
ent primary dust collectors, they were tested at their normal flow
rates and at half of those flow rates. The flow rate was reduced by
decreasing the rotational speed of the vacuum cleaner motor.
The filter bag and cyclonic collectors were tested with potas-
sium chloride (KCl) test aerosol, which is commonly used for dry
filter efficiency testing.
(19)
These test aerosols were also used in
previous studies.
(7,8)
The KCl particles were dispersed by a three-
jet Collison nebulizer (BGI, Waltham, Mass.) from a 0.5% KCl
solution, and were dried by the addition of dry, particle-free air.
Because of their ability to absorb water, salt particles such as KCl
can change in size very rapidly when exposed to environments
with high relative humidity.
(20)
Thus, such particles are not suitable
for evaluating wet collectors. Dry Arizona road test dust, aero-
solized by a Vilnius Aerosol Generator (CH Technologies, West-
wood, N.J.), was used for evaluating the wet collectors. Polydis-
perse Arizona road test dust can be aerosolized as a dry powder
and is typically used to calibrate dust monitors.
(21)
For comparison
purposes the cyclonic collectors were tested with both types of
aerosol.
Measuring the Reentrainment of Particles from Primary
Dust Collectors after Loading with Dust
The dust collection process in a vacuum cleaner with a wet col-
lector is similar to the removal of particles from the sampled air-
stream in a liquid impinger, which is primarily used for sampling
bioaerosol particles.
(22)
In both cases the aerosol is impacted into
a liquid, which bubbles violently as the air escapes and particles
are trapped in the liquid. It has been shown that an impinger is
not only a collector, but also an aerosol generator;
(14,17,18)
that is,
some of the particles collected by the liquid eventually reentrain
into the effluent airflow because of the violent bubbling. In a
vacuum cleaner with a wet collector, a mist separator (fast rotating
vanes) is usually installed above the bubbling liquid to keep the
larger droplets and particles from leaving the wet collector. Testing
was deemed necessary to check for potential passage of already
collected particles through the mist separator. The primary collec-
tors of filter-containing and cyclone-containing vacuum cleaners
were also examined for possible reentrainment of already collected
particles.
At the start of each experiment, the vacuum cleaner was con-
nected through a hose to the filtered air supply system.
(7)
After 10
min, a different hose was connected to the vacuum cleaner and
5 g of Arizona road test dust were delivered to the primary col-
lector by moving the hose inlet over 5 g of the test dust, which
had been distributed over a smooth surface of 400 cm
2
. The pur-
pose of this procedure was to feed the same amount of test dust
into each primary collector being tested in a manner similar to
normal dust pickup in a vacuum cleaner. After all of the 5 g of
test dust was entrained into the vacuum cleaner, the filtered air
supply was reconnected to the vacuum cleaner through a clean
hose. The hose for dust delivery was different from the hose for
the clean air supply to ensure that particle reentrainment after
loading could originate only in the primary dust collector. The
dust delivery operation lasted about 50–55 sec, including 20 sec
for the hose reconnection. The aerosol concentration C
PDC OUT
was
registered by one of the optical particle size spectrometers every
6 sec for 70 min (10 min before test dust loading and 60 min
after the loading).
In earlier studies the authors showed that ambient aerosol may
leak into the vacuum cleaner through potential leak sites in the
nozzle and in the primary filter compartment.
(7,8)
In the present
study, all vacuum cleaners were tested without nozzles to mini-
mize the influence of potential leakage in the nozzle component
on the measured aerosol concentrations in the vacuum cleaner.
The degree of ambient aerosol leakage into the primary filter com-
partment was assessed by measuring C
PDC OUT
before loading the
primary dust collector with test dust while the vacuum cleaner was
connected to the clean air supply system. The aerosol concentra-
tion in the air surrounding the vacuum cleaner being tested was
also monitored before and after each experiment to prove that the
registered changes of C
PDC OUT
after loading with test dust were
not caused by changes in leakage from the ambient air environ-
ment.
RESULTS AND DISCUSSION
Comparison of the Initial Collection Efficiencies for the Different
Primary Dust Collectors
Filter Bag Collection
Figure 2 shows the initial collection efficiencies for the two filter
bags serving as the primary dust collectors in vacuum cleaners
AIHAJ (62) September/October 2001 577
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FIGURE 2. Effect of airflow rate on the initial collection efficiency of the filter bags in vacuum cleaners FC3-UP and FC4-CAN.Tests were conducted
at 100 and 50% of maximum airflow rate through each vacuum cleaner.
FC3-UP and FC4-UP. These tests were performed with KCl test
aerosol. The data presented in Figure 2A are for the filter bag
installed in upright vacuum cleaner FC3-UP. The filtration veloc-
ity through this filter bag, V
F
, was 14.2 cm/sec (;5.6 inches/
sec) at the maximum flow rate through the vacuum cleaner of 60
ft
3
/min. At half of this flow rate, Q
IN
5 30 ft
3
/min, and V
F
5
7.1 cm/sec (;2.8 inches/sec). As can be seen from Figure 2A,
about 72% of the test particles 0.35 to 0.45 mm and more than
98% of the particles larger than 2.0 mm are collected when Q
IN
5
60 ft
3
/min (solid curve with circles). At Q
IN
5 30 ft
3
/min, the
initial collection efficiency for KCl particles is lower in the size
range from 0.35 to about 2.0 mm (dashed curve with triangles).
Such a decrease in collection efficiency with decreasing filtration
velocity is typical for fibrous filters.
(11,13)
The dip in the collection
efficiency curves is due to decreasing particle collection by diffu-
sion and increasing particle collection by impaction and intercep-
tion, as the particle size increases.
(11)
The particle size, d
p
, is the
optical equivalent diameter of KCl particles, as measured by the
optical particle size spectrometer, which was calibrated with stan-
dard polystyrene latex spheres (Bangs Laboratories, Fishers, Ind.).
The collection efficiency curves shown in Figure 2B are for
filter bags installed in the canister of vacuum cleaner FC4-CAN.
At maximum airflow rate, when Q
IN
5 80 ft
3
/min, the filtration
velocity was V
F
5 27 cm/sec (;10.6 inches/sec). The collection
efficiency for test particles smaller than 2.0 mm was lower for the
primary filter collector of vacuum cleaner FC4-CAN (Figure 2B,
solid curve) than for the filter bag of vacuum cleaner FC3-UP
(Figure 2A, solid curve), when the vacuum cleaners were operated
at their maximum flow rate. Particles larger than 2.0 mm were
collected with similar efficiency in both cases. The collection ef-
ficiency of the filter bag in FC4-CAN decreased over almost the
entire monitored particle size range when the flow rate was de-
creased to half of its maximum value (Q
IN
5 40 ft
3
/min, V
F
5
13.5 cm/sec ø 5.3 inches/sec, dashed curve in Figure 2B).
As seen in Figure 2, the collection efficiency of FC3 decreased
less than that of FC4 when the flow rate was reduced to half of
its maximum value. This figure also shows that, at both airflow
rates, the primary filter bag of vacuum cleaner FC3-UP collected
particles more efficiently than the filter bag of FC4-CAN. As in-
dicated earlier, the filter bag of vacuum cleaner FC3-UP consisted
of three layers of fibrous filter material, whereas the filter bag of
FC4-CAN consisted of only one layer. When examined under an
optical microscope, the fiber diameters of the two inner filter layers
of FC3 were found to be noticeably smaller than those of FC4.
The different manufacture of the filter materials and the different
number of filter layers resulted in the higher performance of FC3
versus FC4, although the filtration velocity at maximum airflow
rate for vacuum cleaner FC3-UP was about of vacuum cleaner
FC4-CAN.
Cyclonic Collection
Figure 3 shows the collection efficiencies for the cyclonic collec-
tors in upright vacuum cleaner CC1-UP (Figure 3A) and in the
canister vacuum cleaner CC2-CAN (Figure 3B). Similar to the test
procedure for the filter bag collectors (Figures 2A and B), the
cyclonic vacuum cleaners were also tested at their maximum flow
rates and at half of these values. The solid circles and triangles in
Figures 3A and 3B are for tests with KCl particles. The open di-
amonds and squares in these figures represent the tests with dry
Arizona road dust.
578 AIHAJ (62) September/October 2001
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FIGURE 3. Effect of airflow rate and type of test particles on the initial collection efficiency of the cyclonic collectors in vacuum cleaners CC1-UP
and CC2-CAN
When operated at its maximum flow rate of 50 ft
3
/min (Figure
3A, solid circles), the cyclonic collector of vacuum cleaner CC1-
UP removed less than 40% of 0.5 mm KCl particles. Its collection
efficiency approached 100% only for 4.5 mm and larger particles.
Thus, the cyclonic collector CC1 was less efficient than the filter
bag collectors FC3 and FC4. When the airflow rate through CC1
was decreased to 25 ft
3
/min, the collection efficiency also de-
creased significantly (solid triangles). A decrease in dust collection
at the lower flow rate was expected, because the centrifugal forces
moving particles to the inner wall of the cyclone decrease with
decreasing airflow rate.
(15)
The performance at maximum flow rate
for the cyclonic collector in CC2-CAN (Figure 3B) was much
better, comparable with that of the filter bag in FC3-UP (Figure
2A). The curve with solid circles in Figure 3B shows that about
48% of 0.35 mm KCl particles and close to 100% of KCl particles
larger than 1.0 mm are collected, when vacuum cleaner CC2-CAN
was operated at its maximum flow rate Q
IN
5 40 ft
3
/min. Similar
to CC1, cyclonic collector CC2 also retained significantly fewer
particles over the entire particle size range when the airflow rate
through it decreased (Figure 3B, solid triangles). Comparison of
the distinctly different collection efficiencies for the two cyclonic
collectors demonstrates that construction differences play an im-
portant role in their performance.
The open diamonds and squares in Figure 3 show the collec-
tion efficiencies for these cyclonic collectors when measured with
dry Arizona road dust. The data obtained with Arizona road dust
have greater vertical error bars because of the greater fluctuations
in aerosol concentration when dry dust was dispersed from a pow-
der.
(23)
The performance curves obtained with the two types of
test particles have similar shapes and values for each vacuum clean-
er at the specified flow rates. The small differences are probably
due to the different morphologies and light-scattering character-
istics between KCl particles and Arizona road dust.
(24)
The authors
conclude from the data of Figure 3 that either KCl (dispersed from
a liquid solution) or Arizona road dust (dispersed in a dry form)
may be used to test the collection efficiency of vacuum cleaners.
Arizona road dust data from wet collectors, therefore, can be di-
rectly compared with KCl data from filter-bag or cyclonic collec-
tors.
Wet Collection
Figure 4 shows the collection efficiencies for the wet collectors in
the canisters of vacuum cleaners WC1-CAN and WC2-CAN. In
this case, particles are retained by impinging them into water. Fol-
lowing the recommendations of the manufacturers, the containers
of WC1 and WC2 were filled with 1900 mL (2 quarts) and 3800
mL (1 gallon) of water, respectively. To start the experiments with
particle-free water, only filtered, deionized water was used. The
solid curves with open diamonds in Figure 4 represent the collec-
tion efficiency data for the wet collectors when tested with Arizona
road dust at their maximum flow rates. The dashed curves and
open squares are for half the maximum flow rate. As seen, the wet
collector WC1 removed about 63% of 0.35 mm test particles and
more than 96% of particles larger than 0.7 mm, when Q
IN
5 62
ft
3
/min (Figure 4A). The collection efficiency of WC2 was less
than 60% for 0.35 mm particles, and only particles larger than 1.5
mm were removed with higher than 98% efficiency (Figure 4B).
Thus, the initial filtration efficiency of the wet collector in WC1-
CAN was comparable with that of the filter bag in FC3-UP and
the cyclonic collector in CC2-CAN, when these vacuum cleaners
were operated at their maximum flow rates. The initial filtration
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FIGURE 4. Effect of airflow rate on the initial collection efficiency of the wet collectors in vacuum cleaners WC1-CAN and WC2-CAN
efficiency of the wet collector in WC2-CAN is comparable with
that of the filter bag in FC4-CAN.
Collection efficiency was significantly decreased in both wet
collectors at half of the maximum flow rate (dashed curves in Fig-
ure 4): Only about 30% of particles smaller than 0.50 mm were
collected by the wet collector WC1, and about 10% of these par-
ticles were collected by the wet collector WC2. A decrease in col-
lection efficiency was expected because of the lower force of par-
ticle impingement into water at a decreased flow rate through the
vacuum cleaner. Although a decrease in flow rate is expected in
filter collectors as they become loaded with dust, little change in
flow rate is expected in a wet collector unless a final HEPA filter
is installed and gets loaded significantly. However, decrease of the
liquid level due to water evaporation during vacuum cleaner op-
eration may change the collection efficiency in a wet collector.
Reentrainment of Dust from the Primary Collectors after Loading
with Test Dust
Time Dependence of Dust Reentrainment
Figure 5 shows the aerosol concentrations of dust reentrainment
from the different primary dust collectors during 1 hour after
loading the collectors with 5 g of Arizona road test dust. The total
aerosol concentrations in the size range from 0.3 to about 20 mm,
C
PDC OUT
, were measured in 6-sec time intervals in the air leaving
the primary dust collector. The aerosol concentrations prior to
t50 correspond to the aerosol concentrations measured at the
outlet of the primary dust collector before it was loaded with test
dust. As the test dust was loaded into the primary dust collector
during t50 to 1 min, the aerosol concentration at the outlet of
the primary dust collector, C
PDC OUT
, reached a maximum. During
the subsequent 10 min, the aerosol concentration decreased sig-
nificantly in the effluent flow from each of the dust collectors.
However, the magnitude of dust reentrainment after t510 min
was different for each dust collector: The lowest particle reentrain-
ment was registered for the filter-bag collectors (Figures 5A and
B); it was higher for the wet collectors (Figures 5E and F), and
highest for the cyclonic collectors (Figures 5C and D).
The different initial aerosol concentrations (before t50) reflect
the different levels of ambient aerosol leakage into each collector,
as also shown in a previous publication.
(7)
To ensure that the out-
put concentration, measured after 60 min, is not affected by
changes in ambient aerosol concentration, the latter was moni-
tored before and after each experiment. In all experiments the
average ambient aerosol concentration never changed by a factor
exceeding 1.2 between t50 and 60 min. Figures 5C and 5D show
that the aerosol concentrations at the outlet of both cyclonic col-
lectors 60 min after loading them with 5 g of dust were a factor
of 100 higher than before t50. The measured aerosol concentra-
tions before t50 and at t560 differed by a factor of about 10 for
filter bag collector FC3 (Figure 5A) and for both wet collectors
(Figures 5E and F). These differences can be attributed entirely
to particle reentrainment from the collectors, not to increases in
the ambient aerosol concentration. The time traces shown in Fig-
ure 5 are for single experiments. Similar traces were recorded dur-
ing three repeats for each collector.
Ten minutes after dust loading, the initial level of C
PDC OUT
was
regained only for the filter bag of vacuum cleaner FC4-CAN (Fig-
ure 5B). This indicates that all of the collected dust remained
inside the collector, and none of the previously collected particles
were reentrained after t510 min. Similar performance was ex-
pected for the filter bag of FC3-UP. However, Figure 5A shows
that the aerosol concentration at the outlet of this filter bag was
still about 10 times higher at t560 min than prior to dust loading.
This finding is particularly surprising, because the initial collection
580 AIHAJ (62) September/October 2001
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FIGURE 5. Time dependence of dust reentrainment from different primary dust collectors (PDC) after loading with 5 g of Arizona road test dust
AIHAJ (62) September/October 2001 581
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efficiency of the filter bag in FC3-UP was higher than in FC4-
CAN (Figure 2). Several replicates with vacuum cleaner FC3-UP
resulted in traces similar to the one shown in Figure 5A, even after
all potential leak sites in FC3-UP were sealed with adhesive. Visual
observation confirmed that a considerable amount of the test dust
had penetrated through the filter bag. A layer of dust was found
on the inner walls of the bag compartment, and the color of the
filter bag was darker than before the test. (No change in color was
observed for the filter bag of FC4-CAN.) The color of the filter
bag of FC3 was not uniform, but was interspersed with lighter
areas and spots. This indicates that the dust particles were not
evenly distributed on the inner surface of the filter bag and that
the filtration velocity was not the same across the entire filter me-
dium. One possible explanation for the higher aerosol concentra-
tion at the filter bag outlet after 60 min, compared with the aero-
sol concentration measured before t50 min, is that air turbulence
inside the bag reentrains dust particles, swirls them around, and
then passes some of them through the less than 100% efficient
filter medium.
Sixty minutes after loading, the aerosol concentrations at the
outlets of the cyclonic collectors (Figures 5C and D) were still
about 100 times higher than before loading these collectors with
5 g of dust. The continuous flow of air over the particle deposit
(resulting in aerodynamic drag on the particles) and the impaction
of particles onto the deposits (resulting in scouring) may be the
cause for the high particle reentrainment.
(25)
In both cyclonic vac-
uum cleaners considerable dust deposits were observed on the in-
ner walls of the compartment downstream of the cyclonic collec-
tor.
After the same time period of 60 min, the aerosol concentra-
tions in the outlets of the mist separators downstream of the wet
collectors in vacuum cleaners WC1-CAN and WC2-CAN were
about 30 times (Figure 5E) and 10 times (Figure 5F) higher,
respectively, than prior to dust loading. Since liquid impingers for
aerosol sampling utilize the same collection principle as vacuum
cleaners with wet collectors and have been observed to reaerosol-
ize already collected particles,
(14,17,18)
the authors postulate that al-
ready collected particles in the wet collectors WC1 and WC2 were
reaerosolized through violent bubbling in the liquid reservoir; that
is, the liquid reservoir acted as a dust collector and disperser.
The initial aerosol concentrations measured at the outlets of
both wet collectors were about 10 cm
23
(Figures 5E and F, before
t50). These concentrations (d
p
$0.3 mm) included mineral resi-
dues and water droplets that had passed through the mist sepa-
rator. It was concluded that the increased aerosol concentrations
after the addition of test particles to the water were due to rea-
erosolization of some of these test particles (Figures 5E and F,
t.0). When the liquid reservoir was filled with tap water instead
of filtered, deionized water, the aerosol concentrations measured
at the outlets of the wet collectors were higher; that is, the mineral
residues from evaporated water droplets increased the aerosol con-
centrations.
(26)
The slight increase in aerosol concentration for
WC1 after t530 min was due to the decreasing amount of water
in the collector. Here again, the impinger analogy helps explain
this observation: As the liquid evaporated in an impinger, the re-
maining particles in the liquid were concentrated, resulting in
higher aerosol concentrations in the airflow leaving the imping-
er.
(27)
After about 70 min of operation, the initial water volume of
1.9 L in WC1 was reduced to about 1.3 L. In collector WC2 the
water volume was reduced from 3.8 to 2.9 L.
Particle Size Distributions of Dust Reentrained
after Loading
In Figure 6, the particle size distributions are shown for specific
time periods of the time traces in Figure 5. The beginning of dust
loading corresponds to the first measured time interval of 6 sec
when C
PDC OUT
increased significantly, as registered by the optical
particle size-spectrometer. The curves with solid circles in Figures
6A and 6B represent the particle concentrations registered during
the first minute (t50 to 1 min) after loading with Arizona road
test dust. Because very unstable aerosol concentrations were reg-
istered downstream of the cyclonic and wet collectors right after
loading, the curves for these collectors (solid triangles) represent
the more stable aerosol concentrations measured starting slightly
later, during t50.6 to 1 min (Figures 6C-F). The curves with open
circles correspond to the aerosol concentrations measured during
the second minute (t51 to 2 min); the curves with open squares
are for the sixth minute (t55 to 6 min); and the open triangles
represent the aerosol concentrations measured at the end of the
experiment (t560 to 61 min).
The total aerosol concentrations measured during the first min-
utes after dust loading were higher than 2000 particles/cm
3
for
all collectors. The manufacturer of the optical particle size spec-
trometer recommends this level as the highest aerosol concentra-
tion for measurement with this device. When the aerosol concen-
tration is high, particle coincidence in the view volume of the
device may result in the counting of two or more particles as one,
thus lowering the indicated aerosol concentration. The actual
aerosol concentrations in Figures 5 and 6 may therefore be higher
than shown during the first 5 min. However, since the goal of
these experiments was to semiquantitatively compare the reen-
trainment from the different dust collectors, there was no attempt
to lower the aerosol concentrations by dilution with clean air. If
a correction were applied to the aerosol concentrations during the
first minutes, it would be approximately the same for all collectors,
because the high aerosol concentration registered after loading
was somewhat similar during all experiments (see Figures 5A-F).
As seen in Figure 6A, C
PDC OUT
for the filter bag collector of
FC3-UP decreased more or less monotonically over the entire par-
ticle size range (curves with solid and open circles). The aerosol
concentration measured at t51 to 2 min was about 100 times
lower than the one measured immediately after dust loading. Dur-
ing the next 4 min, C
PDC OUT
further decreased about four times.
From t56 to 61 min, it decreased by an additional factor of about
2. A similar sharp decrease of the aerosol concentration at the filter
bag outlet was measured for FC4-CAN during the second minute
after loading (Figure 6B). In this case, in contrast to the data for
FC3-UP (Figure 6A), the aerosol concentration C
PDC OUT
for par-
ticles smaller than 1.0 mm decreased more rapidly with particle
size. During the first minute after dust loading and throughout
the rest of the experiment, considerably lower aerosol concentra-
tions for particles above 3.0 mm were registered at the filter bag
outlet of FC4-CAN than at the filter bag outlet of FC3-UP. At
the end of the experiment, only particles smaller than 0.7 mm were
reentrained from the filter bag of FC4-CAN.
With both cyclonic collectors (Figures 6C and D) similar de-
creases in C
PDC OUT
were registered during each 60-min test. How-
ever, it can be seen that fewer particles of size larger than 2.0 mm
were reentrained from the cyclone of CC2-CAN than from the
cyclone of CC1-CAN; that is, cyclone CC2 retained more of the
large particles.
The data for the wet collectors (Figure 6E and F) show that
during the second minute after dust loading C
PDC OUT
decreased
more for collector WC1 than for collector WC2. The reverse was
observed between the sixth and sixty-first minutes: C
PDC OUT
de-
creased more for wet collector WC2, resulting in almost the same
C
PDC OUT
levels at the end of the experiment for both wet collec-
tors. In vacuum cleaner WC1-CAN more of the larger particles
582 AIHAJ (62) September/October 2001
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FIGURE 6. Size distributions of particles reentrained from different primary dust collectors at different times after loading with 5 g of Arizona
road test dust
AIHAJ (62) September/October 2001 583
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(d
p
.2.0 mm) were reentrained during the sixty-first minute than
during the sixth minute. This is probably due to the decreased
level of water in the collector of WC1-CAN.
CONCLUSIONS
C
omparison of different primary dust collection methods em-
ployed in vacuum cleaners has shown that the same high initial
collection efficiency can be achieved by either filter bag, cyclonic,
or wet dust collection. For each type of collection device, the col-
lection efficiency depends on the design of the collector. In gen-
eral, the collection efficiency of cyclonic and wet collectors de-
creases more significantly than that of bag filters when the primary
collector and other dust collection components become loaded
with dust and the airflow rate through them decreases. All of the
tested cyclonic and wet collectors were found to reentrain already
collected particles. The amount of reentrainment was lowest for
filter bags.
Based on the limited number of vacuum cleaner models in this
study, one cannot conclude that one method is consistently su-
perior over the others. On the other hand, differences in collection
efficiency curves of individual models within and between method
types were discernible and, in most cases, significant. Preference
of one type of vacuum cleaner over another also depends on the
specific design of the vacuum cleaner, including parameters such
as weight, ruggedness, ease of operation, and the number of fil-
tration elements.
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... The protective effect from vacuuming is unlikely to be due to reporting bias as parents of cases would be likely to over-report cleaning activities rather than under-report. While vacuum cleaners have been shown to be effective collectors and reservoirs of Salmonella from the environment, 47 and have also been implicated in the aerosolisation of contaminated dust, 45,46,48,49 they are often fitted with HEPA filters which reduce the amount of dust recirculating in the air. ...
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Objectives: To determine the prevalence of Salmonella in the environment of case and control houses, and compare serovars isolated from cases and their houses. Methods: From 2005 to 2008, we tested samples from houses of 0-4 year old cases and community controls in Darwin and Palmerston for Salmonella. Case isolates were compared with environmental isolates. S. Ball and S. Urbana isolates were compared using Multiple Amplification of Phage Locus Typing (MAPLT) and Multiple-Locus Variable number of tandem repeat Analysis (MLVA). Results: Salmonella were found in 47/65 (72%) case houses and 18/29 (62%) control houses; these proportions were not significantly different. In 21/47 (45%) houses, case and environmental isolates (from animal faeces, soil and vacuums) were indistinguishable. Multiple serovars were isolated from 20 (31%) case and 6 (21%) control houses. All but one environmental isolate are known human pathogens in the Northern Territory (NT). Each of the four pairs of S. Ball and S. Urbana were indistinguishable. Conclusions: Animal faeces were the most likely source of salmonellosis in cases. The similar prevalence of house isolates suggests that Salmonella is ubiquitous in this environment. The distinction of S. Ball and S. Urbana subtypes enabled linkage of human illness to environmental exposure. Environmental contamination with Salmonella is an important source of sporadic infection in children in the tropics.
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We all know that oxygen is very important for all of us to live a life. Without air we cannot live. On the next hand we also need a fresh air for breathing. Mostly we prefer that there should be fresh air available for breathing. In industry where there is a hazardous environment we cannot breathe properly. We try to reduce the hazardous environment (air pollution) from the working place. In this research work or main objective is to reduce the air pollution (PU Dust) within the working area. The applicator (Roller) is made of polyurethane material. This polyurethane material is light in weight. The grinding process is carried out on the surface of the polyurethane material to remove the damage on the surface of applicator (Roller) and because of which the air pollution takes place. We have designed this dust collection machine for collecting the dust of polyurethane material which is removed during the grinding operation. The inlet of the dust collector is attached near to the carriage of the lathe machine. The Machine will absorb all the material which will be removed during the grinding process. We have used a cyclonic process of air to collect the dust near to the machine. The efficiency of the machine is one of the parameter which is to be validated. The aim of the paper is to represent the design of the dust collection system by using the lapple method which is basically for the machine so that the machine should not look bad and machinist should work in good environment. Through this machine we have achieved the efficiency of 70% theoretically and practically we have achieved the efficiency as 90%. [1]
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Vacuum cleaner is known as a proper way to remove settled dust or aerosols from surfaces to protect building occupants against abiotic and biological particles. In fact, the act of vacuuming the surface re-suspends a significant amount of dust and aerosols in the air. The other source of abiotic and biological particles could be the bag of cleaner and the motor of vacuum cleaner. The bag of the cleaners is the reservoir for microorganisms where they can grow, reproduce and become bio-aerosolized in case of penetration through the cleaner filtration system. Micro-organisms can disseminate from the bag, spread in the system and capture on the final filtration system where overshoot airflow can re-entrain the bioaerosol in the breathing zone which will cause catastrophe for all, especially those who are suffering from allergic and infectious diseases. The motor, due to arcing/abrasion of carbon, emits a significant number of nanoparticles, which can target our cardiovascular and respiratory organs. This review presents a summary of studies on vacuum cleaner and its effect on indoor air quality.
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Over the past few years, human exposure to per- and polyfluoroalkyl substances (PFAS) has garnered increased attention. Research has focused on PFAS exposure via drinking water and diet, and fewer studies have focused on exposure in the indoor environment. To support more research on the latter exposure pathway, we conducted a study to evaluate PFAS in indoor dust. Dust samples from 184 homes in North Carolina and 49 fire stations across the United States and Canada were collected and analyzed for a suite of PFAS using liquid and gas chromatography-mass spectrometry. Fluorotelomer alcohols (FTOHs) and di-polyfluoroalkyl phosphoric acid esters (diPAPs) were the most prevalent PFAS in both fire station and house dust samples, with medians of approximately 100 ng/g dust or greater. Notably, perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), perfluorohexane sulfonate, perfluorononanoic acid, and 6:2 diPAP were significantly higher in dust from fire stations than from homes, and 8:2 FTOH was significantly higher in homes than in fire stations. Additionally, when comparing our results to earlier published values, we see that perfluoroalkyl acid levels in residential dust appear to decrease over time, particularly for PFOA and PFOS. These results highlight a need to better understand what factors contribute to PFAS levels in dust and to understand how much dust contributes to overall human PFAS exposure.
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The changes in the physical collection efficiencies of all-class impingers were studied experimentally with an aerodynamically particle sizer by dynamically measuring the particle concentrations upstream and downstream of AGI-4 and AGI-30 impingers. Monodisperse PSL particles of aerodynamic sizes ranging from 0.3 to 2.0 μm were used in the test. The inner diameter of the impingement nozzle was found to be the most critical dimension affecting the collection efficiency. Significant variations were found in the performance of individual impingers due to the variations in the critical dimensions of the impingers tested. About 1% of the initial amount of impinger liquid is evaporated per minute of normal operation. On depletion to a critical minimum volume, the collection efficiency decrease drastically due to insufficient impingement into the liquid and particle bounce from the bottom surface. Particles already collected may be entrained by the air bubbles passing through the liquid and be reaerosolized, thus decreasing the overall collection efficiency. The number of reaerosolized particles increases with sampling time, but is less than 10% within the first hour of sampling with either the AGI-4 or the AGI-30. A graph has been developed for selecting the maximum sampling time for a given initial liquid volume so that the correction efficiency is approximately constant throughout the entire sampling period. The graph also indicates the limits where significant particle bounce, reaerosolization, or liquid splashing occur. The AGI-4 was found to be more efficient in collecting submicrometer particles and to be less particle-size dependent than the AGI-30.
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The collection efficiency of liquid impingers was studied experimentally as a function of the sampling flow rate with test particles in the bacterial size range. Three impingers were tested: two All-Glass Impingers (AGI-4 and AGI-30), widely used for bioaerosol sampling, and a newly developed slot impinger. The aerosol particles were generated by a Collison nebulizer, and an Aerosizer was used to measure the particle concentrations and size distributions upstream and downstream of each impinger. The effect of the air pressure drop across the impinger on the Aerosizer performance was investigated, and the particle measurement system was modified and calibrated accordingly. While inertial impaction is the dominant particle removal mechanism in impingers, particle bounce and reaerosolization were also found to have significant effects on the impinger collection characteristics. At relatively high flow rates and low levels of collection fluid (corresponding to the collection fluid level after evaporation of most of the liquid during prolonged impingement), the liquid under the impinger jet was observed to be removed by the air pressure and pushed against the container's walls. Particles, such as bacterial or fungal spores, may thus bounce from the bottom of the collection vessel and escape with the effluent air flow or may impact sideways into the liquid that was previously pushed against the walls. It was found that such particle bounce may significantly reduce the collection efficiency of impingers containing a small amount of liquid. When the impingers were operated at a high level of collection fluid and sufficiently high sampling flow rates, it was observed that the bubbles, rising through the liquid, entrained previously collected particles and created new aerosols by bursting at the liquid-air surface. Such particle reaerosolization was also found to reduce the impinger collection efficiency.
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The overall filtration efficiency of a vacuum cleaner traditionally has been tested by placing the vacuum cleaner in a test chamber and measuring aerosol concentrations at the chamber inlet and outlet. The chamber test method was refined and validated in this study. However, this chamber test method shows an overall filtration efficiency of close to 100% for most of the industrial vacuum cleaners and for most of the newly developed household vacuum cleaners of midprice range or higher because all these vacuum cleaners have a high-efficiency particulate air (HEPA) or other highly efficient filter installed at the exhaust. A new test method was therefore developed through which the vacuum cleaner was probed in various internal locations so that the collection efficiency of the individual components could be determined. For example, the aerosol concentration upstream of the final HEPA filter can thus be measured, which permits one to estimate the life expectancy of this expensive component. The probed testing method is particularly suitable for field evaluations of vacuum cleaners because it uses compact, battery-operated optical particle size spectrometers with internal data storage. Both chamber and probed tests gave the same results for the aerosol filtration efficiency. The probed testing method, however, also gives information on the performance of the individual components in a vacuum cleaner. It also can be used to determine the dust pickup efficiency and the degree of reaerosolization of particles collected in the vacuum cleaner.
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A new aerosol generator is introduced in which particles suspended in a liquid are aerosolized by gentle bubble bursting. Tangential injection of dry air to the bubbling surface dries the airborne droplets immediately after aerosolization so that they rapidly shrink in size and are carried out from the generator by inward and upward swirling air motion. The new generator has been evaluated with monodisperse PSL particles in the size range of 0.73–5.1 μm and with a saline solution using a time-of-flight aerodynamic particle size spectrometer (Aerosizer). It was found that, in contrast to pneumatic nebulization (e.g., with a Collison nebulizer), the new generator's output in undesirable liquid droplets is very small, while its output in dry PSL particles is high. When using the new aerosol generator, a minimum number of the nebulized droplets is returned to the liquid pool, thus optimizing the number of particles available as test aerosols. The aerosol concentration was found to be constant and stable for at least 30 min with the prototype generator tested. It is shown that the relative humidity of the effluent flow can be regulated. For microorganisms aerosolized by this generator, the shear stress is expected to be considerably lower than in conventional aerosol generators.
Conference Paper
Ambient aerosols frequently contain large proportions of hygroscopic inorganic salts such as sulfates and nitrates, which may induce adverse health effects upon inhalation. The inhaled salt particles are invariably exposed to a humid environment; their deposition along the respiratory tract will necessarily depend upon the size change resulting from water vapor condensation. This paper discusses the deliquescent properties of pure and mixed salt aerosols and the particle size change as a function of relative humidity. Experimental results are presented for the growth of mixed chlorides (NaCl-KCl), mixed sulfates (H{sub 2}SO{sub 4}-(NH{sub 4}){sub 2}SO{sub 4}), and mixed (NH/sub 4/){sub 2}SOsub 4}-NH{sub 4}NO{sub 3} aerosol systems. It is shown that the behavior of the mixed salt aerosols in a moist atmosphere can be predicted from phase diagrams and pertinent thermodynamic properties of the bulk solutions. The evaporation of a saline droplet in an atmosphere of decreasing humidities is also investigated experimentally. For each deliquescent salt aerosol, there is a threshold humidity below which the solution droplets will quickly evaporate to become crystalline particles. The information is useful in the selection of a suitable humidification procedure to generate test aerosols for exposure studies.
A cyclone with a 47 mm after-filter has been developed for ambient air size-selective monitoring. It has been extensively evaluated with laboratory-generated aerosol. Variation of the pressure drop and 50% cut point with flow rate show that the cyclone operates in a single flow regime with a vortex in the outlet flow. The particle size cutoff curve is comparable in sharpness to a cascade impactor and is the same for solid or liquid particles. At 21.7 L/min, D50 is 2.5 μm and at 15.4 L/min, D50 is 3.5 μm. Collection efficiency data for flow rates from 8 to 27 L/min fit a universal curve when plotted vs. the normalized particle diameter, (D-D50)/D50. Reentrainment of previously deposited particles is less than 1% of the loading per day. In field tests the cyclone has proved to be a very satisfactory size-selective sampler.
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A new principle for collecting airborne particles, including microorganisms, has been introduced by injecting the particles into a swirling airflow from where they are removed onto a collection surface. A dry surface, a surface coated with an adhesive substance or a surface wetted by a liquid swirled onto the collection surface from a reservoir below can be used in the new collection method. The swirling air motion and aerosol injection into it are achieved by drawing the airborne particles through nozzles that are directed at an angle toward the collection surface. This principle has been incorporated into a new sampler that has been named “Swirling Aerosol Collector” (SAC; commercially available as the “BioSampler” from SKC Inc., Eighty Four, PA). The physical performance of the SAC has been evaluated against the widely used AGI-30 impinger by measuring the particle concentrations upstream and downstream of each sampler with an aerodynamic particle sizer. Tests with monodisperse polystyrene latex (PSL) particles ranging from 0.3 to 2.0 μm have shown that the SAC has better collection efficiency than the AGI-30 when the same collection liquid is used. A conventional impinger maintains constant collection efficiency for a relatively short sampling period, as the liquid evaporates quickly due to the violent bubbling of the liquid. In contrast to conventional impingers, the SAC can be used with nonevaporating liquids that are considerably more viscous than the liquids used in the impingers. Thus, the SAC can sample over any period of time. The new aerosol sampler produces minimal or no reaerosolization of particles collected in the liquid in contrast to significant reaerosolization in a conventional impinger. Since the SAC projects the aerosol particles toward the collection surface where they are removed from the swirling flow, it avoids or significantly reduces particle bounce from the collection surface even when the surface is dry.
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The Collison nebulizer is widely used to produce fine aerosols from a liquid supply. Details of its design and operating characteristics are given, including air and liquid consumption, aerosol output rate and droplet size distribution. An adaptor for the outlet of the nebulizer is also described. This is used when monodispersed aerosols are being generated and enables the output of particles to be increased.